Move the night way: how can physical activity facilitate adaptation to shift work?

Shift work, involving night work, leads to impaired sleep, cognition, health and wellbeing, and an increased risk of occupational incidents. Current countermeasures include circadian adaptation to phase shift circadian biomarkers. However, evidence of real-world circadian adaptation is found primarily in occupations where light exposure is readily controlled. Despite this, non-photic adaptation to shift work remains under researched. Other markers of shift work adaptation exist (e.g., improvements in cognition and wellbeing outcomes) but are relatively unexplored. Timeframes for shift work adaptation involve changes which occur over a block of shifts, or over a shift working career. We propose an additional shift work adaptation timeframe exists which encompasses acute within shift changes in markers of adaptation. We also propose that physical activity might be an accessible and cost-effective countermeasure that could influence multiple markers of adaptation across three timeframes (Within Shift, Within Block, Within Work-span). Finally, practical considerations for shift workers, shift work industries and future research are identified.

The perspective piece by Easton, Gupta, Vincent and Ferguson is an interesting and timely discussion on a potential role for physical activity to be used as a countermeasure for the negative consequences of shiftwork.The approach the Authors take, placing their hypothesis in the broader context of an appreciation of the holistic nature of circadian biology, was refreshing and certainly appropriate.Overall, there was a good consideration of pros/cons, as well as a detailed discussion of some of the potential hardships/barriers to adopting some of these approaches.The science that is cited is a mix of older and newer sources, and from this reviewers' perspective seem appropriate.My few comments below are considered minor writing changes to improve clarity and preciseness.
As this was a perspective/review piece, there was no primary data to be assessed.
Ln141 "Thus, this paper aims discusses…" seems like it should read "Thus, this paper aims to discuss…" Ln148 I would suggest the Authors make a modification to note the circadian system is comprised of central clock in the suprachiasmatic nucleus of the hypothalamus (with some references), and many endogenous central and peripheral oscillators.
This would provide a more accurate illustration of the circadian system as a coordinated set of oscillators driven by a central clock.
Ln151 I suggest the Authors edit this section a bit as well, as it is a bit unclear.Circadian rhythms are rhythms that occur with a periodicity of about 24 h, not just "activity" which is the way this.The next thought could then be a bit clearer that examples of such rhythms in physiology and behavior include many processes such as activity, sleep, core body temperature, melatonin, and cognitive performance.Ln160 I would add "not as strong, at least in humans and most vertebrates" since in insects and plants they can be.I know that this article focuses on humans, but it is important to make this clear for a more general audience.
Ln161 I don't really understand this sentence.Circadian timing can't be "determined" by measuring biomarkers.Maybe the Authors mean that the phase of the circadian clock can be determined by measuring biomarkers?If this is the case, then they should edit this to read that these physiological outputs are controlled by outputs from the circadian system and also by external signals… something like that would clear up things.

Reviewer #3 (Remarks to the Author):
Thank you for the opportunity to review this interesting and comprehensive review paper.
1.The paper offers a strong foundation for the need for further work in shiftwork adaptation support.More significantly, the authors propose some interesting and novel ideas about both markers and measurement frames for adaptation.Specifically, the authors propose that shift work adaptation be viewed through a holistic lens that expands to all biopsychosocial markers of adaptation.In addition, the authors propose a novel third timeframe of shift work adaptation, "within shift", setting out three timeframes in which shift work adaptation may occur and offering time-specific mechanisms and markers of adaptation.Figure 1 was particularly creative.
2. Overall, I found the paper to be well written and well supported.The arguments were well developed and logically organized.The paper was creative and offered important and novel insights and help to map out future areas of research.I offer a few relatively minor edits and comments for your consideration below.
3. P. 4. The opening line of the paper claims "The current demands…", which is true, but it is supported by a 2003 reference which undermines the strength of your claim.

Response to Reviewer
The stated objective of the review article by Easton et al is to discuss the possibility that physical activity can serve as a non-photic adaptation signal in shift workers, for improvement of sleep, cognition, health and wellbeing, as well as to decrease risk of occupational incidents.Overall, this is an interesting article, which highlights information on shift work, and proposes an interesting hypothesis/concept.The purpose of the following comments/questions are to help the reader understand supporting evidence for the proposed hypothesis/concept.Thank you for taking the time to review the manuscript and for your suggestions for improvement.#1.Within the review article, the terms circadian rhythms and alignment/misalignment are frequently used.However, the Reviewer was unable to find full/accurate definitions of these terms.For circadian rhythms, the authors state that 'Circadian rhythms are any process which follows a 24 h cycle…'.With regards to alignment/misalignment, please provide a definition (whether this is with respect to alignment of the organism with the environment, or between cells/organs within an organism).
We agree these definitions are needed throughout the manuscript.We have revised this section of the introduction to further emphasise the significant impact of shift work.
Please see our response to Reviewer #2, comment #2 and #3 which have been amended to highlight the definition of circadian rhythms: Lines 165-169: "The circadian system is comprised of a central clock located in the suprachiasmatic nucleus of the hypothalamus, connected to other endogenous peripheral oscillators (Aschoff, 1960;Weaver, 1998).The cellular and molecular interactions of these central and peripheral oscillators collectively generate circadian rhythms and synchronise rhythms to the light-dark cycle (Aschoff, 1960;Kryger, 2000)." Lines 169-172: "Circadian rhythms are rhythms which occur with a periodicity of 24 h (Aschoff, 1960).These rhythms include behavioural and physiological processes such as sleep, core body temperature, melatonin, cognitive performance and alertness (Cagnacci et al., 1997;Dijk et al., 1992;Muck et al., 2022)." Lines 59-64 have also been amended to define circadian misalignment: "Such negative consequences of shift work result from circadian misalignment, which broadly is a term that refers to the mismatch between the timing of sleep and wake, the light dark cycle, and other circadian rhythms (i.e., behavioural, physiological, hormonal, cellular) (Resuehr et al., 2019).As such, misalignment can be internal (i.e., rhythms not aligned with each other) and/or external (i.e., rhythms not aligned with environment) which has implications for health and wellbeing."#2.The Reviewer recommends highlighting why alignment/misalignment is important, in terms of health and disease.In the absence of such background information, it may be difficult for the Reader to understand why this needs to be 'fixed' in shift workers.
We agree that this is an important addition to the paper.2003).Though this study could not determine whether a phase advance or a delay occurred, these data suggest that physical activity may contribute important timing information for the synchronisation of circadian clocks throughout the body.Taken together, human studies of hormonal and cellular rhythms suggest that physical activity may align various circadian and peripheral clocks and could assist in the adaptation to shift work.Thus, while additional research is needed to elucidate the appropriate timing for physical activity to elicit a phase delay of hormonal and cellular rhythms in shift workers, physical activity may be a plausible strategy to assist Within Block adaptation."#5.As mentioned in the article, a large number of zeitgebers exist, some of which tend to influence circadian clocks in tissue selective manners.As such, it may be important to align zeitgebers in an appropriate manner, for optimal circadian biology.The authors focus on physical activity and light.Another, which is likely highly relevant for shift workers, is food intake.For example, recent work by Panda and colleagues has shown benefit of time restricted feeding in firefighters.
The authors agree that many zeitgebers contribute to optimal circadian biology.The focus of the review is physical activity.However, we agree that highlighting the importance of optimising many other external zeitgebers is necessary to provide a comprehensive understanding of how to best support shift workers.
Lines 464-476 demonstrate the adjusted section to further highlight the interaction between other aspects that support circadian adjustment: "Strategies to align these behaviours with the light-dark cycle have benefits for cardiometabolic health.For instance, timerestricted eating is a method which shortens the eating window to optimise the timing of food intake for reduced cardiovascular disruption, and therefore reduced risk of cardiovascular disease (Gupta et al One topic that is only briefly mentioned in the article is 'days off'.Shift work is a very heterogenous profession, with many different types of shifts.Some can be more acute than others (e.g., 4 night shifts, 3 days off).The Reviewer is curious regarding the authors thoughts on whether it is better for shift workers to maintain (as much as possible) their night shift schedule (and therefore circadian rhythms) during their days off.The importance of this lays with the question as to whether it is better to realign circadian clocks back and forth as quickly as possible, or to maintain a constant alignment (i.e., no shift).Such a standpoint will influence 'when' exercise should be performed not only during the night shift, but also during the days off.
As the reviewer notes, the authors cannot provide a one-size fits all recommendation as to whether workers maintain a delayed circadian rhythm, given the many requirements outside of the work environment.In an ideal world, maintaining an adjusted rhythm for night work would be the better option for circadian adaptation, but not for other aspects of adaptation, such as improved family and social interactions.It is generally not feasible to maintain a night shift schedule because light exposure cannot be managed in or outside of work.The timing of physical activity should then be optimised to assist overall adaptation to shift work.
Lines 619-624 have been amended to reflect this notion."Circadian adaptation to long periods of night work is generally not feasible in naturalistic environments given that light exposure cannot be managed during or outside of work (Dumont et al., 2001;Ferguson et al., 2012).However, nonphotic entrainment by way of physical activity may be beneficial on consecutive night shifts if the timing of physical activity can be optimised to assist overall adaptation to shift work, and the shift working schedule."#7.The topic of chronotype and shift work is fascinating.It is recommended that the author consider mentioning the Nurses Health Study results (PMC4542269).
Thank you for providing further evidence to strengthen this section of the paper.We have included evidence from this study in lines 581-586 which say: The perspective piece by Easton, Gupta, Vincent and Ferguson is an interesting and timely discussion on a potential role for physical activity to be used as a countermeasure for the negative consequences of shiftwork.The approach the Authors take, placing their hypothesis in the broader context of an appreciation of the holistic nature of circadian biology, was refreshing and certainly appropriate.Overall, there was a good consideration of pros/cons, as well as a detailed discussion of some of the potential hardships/barriers to adopting some of these approaches.The science that is cited is a mix of older and newer sources, and from this reviewers' perspective seem appropriate.My few comments below are considered minor writing changes to improve clarity and preciseness.
We thank the Reviewer for their positive comments on the manuscript and for providing constructive feedback and considerations.
#1. Ln141 "Thus, this paper aims discusses…" seems like it should read "Thus, this paper aims to discuss…" Thank you for pointing out this grammatical issue.This has been rectified in lines 158-160: "Thus, we aim to discuss several markers of shift work adaptation…" #2.Ln148 I would suggest the Authors make a modification to note the circadian system is comprised of central clock in the suprachiasmatic nucleus of the hypothalamus (with some references), and many endogenous central and peripheral oscillators.This would provide a more accurate illustration of the circadian system as a coordinated set of oscillators driven by a central clock.
The authors agree that this should be clarified within this paragraph to provide a comprehensive overview of the circadian system.Please see lines 165-169: "The circadian system is comprised of a central clock located in the suprachiasmatic nucleus of the hypothalamus, connected to other endogenous peripheral oscillators (Aschoff, 1960;Weaver, 1998).The cellular and molecular interactions of these central and peripheral oscillators collectively generate circadian rhythms and synchronise rhythms to the light-dark cycle (Aschoff, 1960;Kryger, 2000)."#3.Ln151 I suggest the Authors edit this section a bit as well, as it is a bit unclear.Circadian rhythms are rhythms that occur with a periodicity of about 24 h, not just "activity" which is the way this.The next thought could then be a bit clearer that examples of such rhythms in physiology and behavior include many processes such as activity, sleep, core body temperature, melatonin, and cognitive performance.
Thank you for highlighting this, we have adjusted this section to ensure that this section does not misconstrue the intricacies of the circadian system.Please see adjusted lines 169-172: "Circadian rhythms are rhythms which occur with a periodicity of 24 h (Aschoff, 1960) Thank you for the opportunity to review this interesting and comprehensive review paper.
#1.The paper offers a strong foundation for the need for further work in shiftwork adaptation support.More significantly, the authors propose some interesting and novel ideas about both markers and measurement frames for adaptation.Specifically, the authors propose that shift work adaptation be viewed through a holistic lens that expands to all biopsychosocial markers of adaptation.In addition, the authors propose a novel third timeframe of shift work adaptation, "within shift", setting out three timeframes in which shift work adaptation may occur and offering time-specific mechanisms and markers of adaptation.Figure 1 was particularly creative.
We thank the reviewer for their positive feedback on this manuscript.
#2.Overall, I found the paper to be well written and well supported.The arguments were well developed and logically organized.
The paper was creative and offered important and novel insights and help to map out future areas of research.I offer a few relatively minor edits and comments for your consideration below.
Chronotype is consistently studied within laboratory and field settings (Colelli et al., 2023; Harfmann et al., 2020; Merikanto et al., 2013; Vetter et al., 2015) with field And in earlier lines 130-136: "To date, these strategies have largely involved a focus on circadian adaptation.Circadian adaptation is typically defined as the process by which rhythms move to partially or fully match a new externally imposed schedule (e.g., time zone when travelling, or night shift when working) -otherwise referred to as phase shifting.Circadian phase shifts are defined and measured using markers of timing (e.g., onset, offset, peak, nadir) of core physiological rhythms, such as sleep/wake, melatonin, core body temperature and cortisol(Chang et al., 2013; Costa et al.,  2014; Kripke et al., 2007; Roden et al.,